The invention relates to machines for measuring the electrical resistivity of bulk materials; and in particular to a machine for the automated, multi-point, multi-surface measurement of the resistivity of a semiconductor boule.
The four-probe technique to measure electrical resistivity was proposed for semiconductors as early as 1954. This principle of measurement was known much earlier than that and was used to measure the resistivity of the earth. The resistivity of the sample measured is given by the following equation 1,   ρ  =      2    ⁢          xe2x80x83        ⁢    π    ⁢          xe2x80x83        ⁢    s    ⁢          xe2x80x83        ⁢          F      ⁡              (                  V          I                )            
where:
xcfx81 is the resistivity measured by a collinear four point probe
s is the spacing of the probes
F is the correction factor
V is the measured voltage
I is the source current
The correction factor accounts for several variables: the thickness of the sample under investigation, location of the probe from the wafer edge, diameter of the sample and location of the current and voltage probes. In addition there is a correction for temperature.
The correction factor depends on the probe spacing to the specimen diameter, the specimen thickness, the slice thickness to probe spacing and the probe tip correction factor. Under the circumstance in which one is not measuring a thin specimen ( less than 1.6 mm) and if the probes are far away from the edge of the specimen which has a large diameter ( greater than 16 mm), there would be no correction factor. Equation 1 would reduce to equation 2:   ρ  =      2    ⁢          xe2x80x83        ⁢    π    ⁢          xe2x80x83        ⁢    s    ⁢          xe2x80x83        ⁢          (              V        I            )      
The probes can be spaced in such a way that 2 xcfx80s=1, s being measured in centimeters. The bulk resistivity equation would be simply equation 3:   ρ  =      (          V      I        )  
It is to be noted that temperature plays an important role. Therefore corrections should be made for the measured temperature. It is also important to choose the magnitude of the current to be used for measurement to prevent minority carrier injection as well as localized sample heating.
When measurement of resistivities on boules are made, equation 3 becomes the control equation since one is measuring bulk resistivity at all times.
The measurement of resistivity of semiconductor boules is presently a time-consuming task, difficult to monitor for accuracy of data and position of test points and vulnerable to mistakes.
It is a goal of the invention to provide an apparatus with a fully automatic measurement system for the four probe measurement and recordation of electrical resistivity characteristics of a semiconductor boule.
It is an objective of the invention to make the fully automatic measurement system adaptable to a production line environment for sequential delivery and removal of boules to a topside boule support grid thereon.
It is a still further objective to make the fully automatic measurement system adaptable to different diameter boules, and to boules of different lengths.
It is another objective to provide the fully automatic measurement system in the form of a shielded apparatus, so as to protect moving parts, cables and hoses as much as possible from exposure to the boule and transport mechanisms.
It is an additional objective to provide a mechanized, multi-point probe head, resistivity sensor system with simple multi-axis motion capability with both linear and rotational components, that can be operated on an automated basis for full cycle surface mapping, resistivity measurements, and recording of all data.
It is still yet another objective to provide an apparatus that provides for centerline movement of a probe head carriage assembly between boule ends and below the boule.
Other goals and objectives within the scope of the invention will be readily apparent to those skilled in the art, from the figures and description of preferred embodiment that follows.